In liquid crystal display devices, speed-up of a display speed is now requested, and multiplex driving of the liquid crystal display devices is also requested. Further, in optical switching elements, speed-up of an optical switching speed is requested.
On that account, regarding to light reflection type liquid crystal elements wherein an intensity of a reflected light from a liquid crystal element surface such as a display panel of a liquid crystal display device is electrically changed, it is desired that the electrooptical change of the reflected light intensity can be made at a high speed. Also in the case of using a liquid crystal element as the optical switching element, it is desired that an electrooptical change of an intensity of a transmitted light passing through the liquid crystal element can be made at a high speed.
However, when the light reflection type liquid crystal element or the light transmission type liquid crystal element is a liquid crystal element of TN type or STN type, this liquid crystal element is long in the response time required for the electrooptical change, for example, the response time is about several millisec. to several tens millisec.
In order to shorten the response time of the liquid crystal element to not longer than several tens microsec., use of a ferroelectric liquid crystal material instead of a nematic liquid crystal material in the liquid crystal element of TN type or STN type has recently been tried.
In the liquid crystal element using the ferroelectric liquid crystal material, spontaneous polarization of the ferroelectric liquid crystal compound is utilized to give rise to the electrooptical change.
If a voltage is applied to the above-mentioned liquid crystal element to arrange a direction of the spontaneous polarization of the ferroelectric liquid crystal compound in a given direction and to maintain this state, an internal electric field is formed inside the ferroelectric liquid crystal material owing to the polarization. This internal electric field hinders the ferroelectric liquid crystal material from electrical transference of from a certain state to other state. For this reason, after a voltage is applied to the liquid crystal element containing the ferroelectric liquid crystal material for a long period of time, the intensity of the reflected light from the liquid crystal element surface or the intensity of the transmitted light passing through the liquid crystal element cannot return to its initial state when a reverse voltage is applied thereto, as far as the level of the reverse voltage is low. Thus, the liquid crystal element using the ferroelectric liquid crystal material has a drawback of switching failure.
In contrast therewith, a liquid crystal element using an antiferroelectric liquid crystal material has been proposed recently, and an improvement of a drawback of the liquid crystal element using the ferroelectric liquid crystal material, for example, the above-mentioned switching failure, has been tried.
However, the liquid crystal element using the antiferroelectric liquid crystal material has no optical memory properties, and hence if application of a voltage to the liquid crystal element is stopped, the intensity of the reflected light from the liquid crystal element surface or the intensity of the transmitted light passing through the liquid crystal element returns naturally to its initial state. Therefore, when the antiferroelectric liquid crystal material is used for a display panel of a liquid crystal display device, there is involved such a drawback that a displayed image cannot be retained unless a bias voltage is applied to the material.
In the liquid crystal element using the ferroelectric liquid crystal material or the antiferroelectric liquid crystal material, orientation properties of the liquid crystal material filled between electrodes of the liquid crystal element are deteriorated at the central portion between the electrodes, unless the distance between the electrodes is adjusted to not more than 2 .mu.m. Hence, the liquid crystal element using the ferroelectric liquid crystal material or the antiferroelectric liquid crystal material has such a drawback that the response time cannot be shortened to not longer than several tens microsec.
In the liquid crystal elements of conventional types, namely, the TN type liquid crystal element, the STN type liquid crystal element and the liquid crystal element using the ferroelectric liquid crystal material, an intensity of a specific polarized light (i.e., polarized light having a fixed wave vector) contained in the reflected light from the liquid crystal element surface or an intensity of a specific polarized light contained in the transmitted light passing through the liquid crystal element is electrically changed utilizing the refractive index anisotropy or birefringence of the liquid crystal material contained in the liquid crystal element. For example, in the case of using any of the conventional type liquid crystal elements as an optical switching element, two polarizing plates (first and second polarizing plates) are generally provided before and behind the liquid crystal element, and a light passes through the first polarizing plate and the second polarizing plate successively to output only a specific polarized light.
Accordingly, in the conventional type liquid crystal elements, an optical loss of about 75% of the incident light usually occurs because of the two polarizing plates, and hence a sufficiently high change of light intensity cannot be obtained unless a powerful light source is used.
Further, in the case of using the conventional type liquid crystal elements, a large-sized display screen is hardly obtainable, so that they are unsuitable for light-adjusting sheets, light-adjusting glasses and large-sized screen displays.
Furthermore, when the liquid crystal material for the liquid crystal elements is made only of a liquid crystal compound, the production cost of the liquid crystal elements incorporating such liquid crystal material is high because the liquid crystal compound is expensive.
In order to eliminate the above-mentioned drawbacks, liquid crystal materials comprising a liquid crystal compound and an organic polymer have been recently proposed. For example, a liquid crystal material comprising a ferroelectric liquid crystal and an organic polymer is proposed in Japanese Patent Laid-Open Publication 260841/1987. Such liquid crystal materials comprise a liquid crystal compound and an organic polymer which is cheaper than a liquid crystal compound, and hence they are available at a lower price as compared with the liquid crystal materials made of only a liquid crystal compound.
In these liquid crystal materials, the organic polymer functions as a matrix for the liquid crystal compound, whereby the liquid crystal materials can be made in the form of a film. If a liquid crystal material in a form of film is used, a liquid crystal element having a large surface area can be readily prepared. Further, the liquid crystal element prepared by using the filmy liquid crystal material is very suitable for a light-adjusting sheet, a light-adjusting glass and a large-sized screen display.
However, the liquid crystal element using the liquid crystal material in a form of film, such as a liquid crystal element using a liquid crystal material comprising a ferroelectric liquid crystal compound and an organic polymer, has a drawback inherent in ferroelectric liquid crystal materials, for example, occurrence of switching failure.
In general, a difference in the refractive index between the liquid crystal compound and the organic polymer is large, and, therefore, a light scattering at the interface between the liquid crystal compound and the organic polymer occurs due to the difference in the refractive index. Accordingly, when the liquid crystal material of this kind is used for a display panel of a projection type large-sized screen display, there is involved such a drawback that an optical difference between the transparent state and the light-scattering state, namely, contrast, cannot be-made high.
The conventional liquid crystal element using a liquid crystal material comprising a liquid crystal compound and an organic polymer becomes a light-scattering state when no voltage is applied thereto, but becomes a transparent state when a voltage is applied thereto. Accordingly, for example, in a vehicle in which the liquid crystal element of this kind is used for a light-adjusting window glass, if an electric power is stopped for some reason or other at the time of applying a voltage to the liquid crystal element, the light-adjusting window glass becomes the light-scattering state to show opaque, resulting in a problem of safety.